They wouldn't even if they could. They're desperate to maintain the illusion that the UK will be able to escape from any commitments or obligations to the EU and still be given a free trade agreement. No contribution to the EU budget, no need to comply with any directives, especially the free movement of labour, but still trade as before.
The problem for the Brexiters is that they cant have any sensible debate before the vote about alternative forms of UK - EU relationship because they would have to accept that some form compromise would be required. And there's no chance of the various Leave groups agreeing on what the red lines would be (other than that everything would be a red line).
The risk is that if we vote Leave it would then take several months of internal squabbling before they could agree on a mandate for the negotiators to open negotiations. All that will happen is that Boris will ask for a free trade agreement with no strings attached, be politely told by Merkel to get serious, and then there will be a Mexican stand off until the second referendum.